The present invention relates generally to the art of electric motors, which should be understood as described herein to include generators and other electromechanical machines which effect a conversion between mechanical and electrical power. More particularly, the invention relates to an arrangement for providing improved air flow within an electric motor.
Rotors of electric motors often include areas in which pressure systems are created during the motor's operation. For example, in a motor having a fabricated bar rotor, conductive bars, for example made from aluminum or copper, extend through the rotor core parallel to and radially spaced from the rotor central axis. The rotor bars are equally spaced apart from each other to form a "squirrel cage" arrangement. The ends of the bars extend beyond either end of the rotor core. Each end of these bar extensions are electrically connected by a conductive annular ring, for example made from the same conductive material as the bars. A fan is typically disposed on the shaft outboard of each annular ring. The fan generally has an annular base portion extending radially outward from the shaft.
Thus, a gap is defined between the ends of the rotor core and each annular fan base portion. Each gap is radially bounded by the bar extensions. As the rotor rotates, negative pressure is created at the inner diameter of the bar extensions tending to draw air from the gap radially outward to positive pressure at the rotor outer diameter.
Generally, however, these negative pressures are unable to draw air into the vacuum to then push out to the rotor outer diameter. For example, the fan base portions are typically solid, possibly except for bore holes to provide access to bolts holding the rotor core together when the core is constructed as a stack of annular steel laminations. These relatively small bore holes, however, do not generally permit air flow into the low pressure vacuum gaps.
Such air flow may be prevented, for example, by pressure drop losses and windage friction. Pressure drop losses result primarily from restricted air flow outlets from the stator. Air pushed radially outward from the fan blades generally flows either through a gap between the rotor and the stator or around the coil head windings. Air flowing through the air gap between the rotor and the stator exits the stator through stator ducts. Air flowing around the coil head windings exit a side exhaust. Because of the compact winding arrangement, the stator ducts' size is limited. Consequently, the alternate air flow path over the coil head windings to the side exhaust is designed to achieve an appropriate pressure drop to force air through the stator/rotor air gap to cool the motor core. This restricts the overall air flow path out of the motor and provides resistance to air that might be pushed from within the cage inner diameter by the negative pressure. This, in turn, reduces the ability of the negative pressure to draw air into the cage inner diameter through the bore holes. Windage friction, which is friction between air and the fan base portion face proximate the bore holes, also inhibits air flow into the gaps.
Since no air is drawn into the gaps, the negative pressure at the cage inner diameter does not contribute to the air flow that cools the motor. While it should be understood that a negligible amount of air may pass through the bore hole of such fans, such air flow, if it occurs, does not measurably increase air flow within and out of the motor as measured by standard methods. Thus, for purposes of the present disclosure, any such air flow is considered nonexistent.